Use `apk --no-cache' on alpine images (#144)

This enables us to remove the following patterns that required
a `rm -rf /var/cache/apk`:

- `apk update`
- `apk add --update`
- `apk add --update-cache`

Supported since alpine 3.3.

gcalcli/Dockerfile

@@ -3,12 +3,11 @@ MAINTAINER Jessica Frazelle <jess@docker.com>
 
 ENV HOME /home/gcalcli
 
-RUN apk update && apk add \
+RUN apk --no-cache add \
 	python \
 	python-dev \
 	py-pip \
 	build-base \
-	&& rm -rf /var/cache/apk/* \
 	&& adduser -S gcalcli \
 	&& chown -R gcalcli $HOME \
 	&& pip install vobject parsedatetime gcalcli